First off - let's get one thing crystal clear! The CAA "asks" airlines is not the same thing as a "rule" or a "guideline" at all. It is only a suggestion!
Secondly, the CAA has no balls - otherwise Ryanair would have been grounded within weeks of commencing its UK operations over the breaches of safety regarding "utterance of false statements to obtain ATC clearance". (In their early days, their pilots routinely reported falsely that they were "low on fuel" in order to jump the queue for landing - a serious safety offence as it could make Air Traffic Controllers disbelieve a genuine "low fuel" alert!) Ryanair, EasyJet and all US carriers operating out of the UK would have been grounded for overloading cabin baggage lockers sometimes to twice or three times their CAA plated weights etc etc etc
For your information, NorthWest is to introduce a fixed charge for seating together, or for an Emergency Exit seat, or for a Window or Aisle seat. The only seat free of any charge will be a middle one!